In order to meet great demands of transmission capacity with respect to an optical communication system, a wavelength multiplex transmission system has been developed. The capacity of wavelength multiplex transmission system is determined by a product of transmission capacity per wavelength and a wavelength number; for this reason, it is desired to increase a transmission capacity per wavelength and the wavelength number.
A practical use by 10 Gbps has been so far performed as a transmission capacity per wavelength, and the study and development have been made in order to realize a 40-Gbps transmission capacity. However, there is a limit in a response speed of an optical modulator, an optical modulator driver, an optical electronic device such as photo-diode, electronic circuits; for this reason, it is very difficult to increase the response speed. Thus, it is greatly expected to develop a communication method having no limit of device response speed. As one of the communication method, there is an amplitude modulation multi-valued coding communication method (hereinafter, referred to “multi-valued modulation method”).
For example, in the case of a binary 10 Gbps signal, a time slot width given to one bit is 100 ps. When two 10 Gbps binary codes are converted into one quaternary code, two bits are represented in the same time slot width 100 ps; therefore, it is possible to realize a 20-Gbps transmission capacity without increasing a device response speed.
Moreover, when four 10 Gbps binary codes are converted into one hexadecimal code, four bits are represented in a time slot width 100 ps; therefore, it is possible to realize a 40-Gbps transmission capacity. As described above, the multi-valued modulation method is a method effective for increasing a transmission capacity without receiving the limit of device response speed.
In order to increase a wavelength number in the wavelength multiplex transmission system, an interval between adjacent wavelengths must be made narrow. To give an example of the factor of limiting the wavelength interval, there are a stability of light source, a wavelength accuracy of optical composing/decomposing unit, and a modulation spectral width. The modulation spectral width of these factors is an essential problem. A half width of modulation spectrum is given by the reciprocal of time slot width.
For example, when a 10-Gbps binary code is converted into an optical intensity modulation signal, an optical spectral width is broadened into about 20 GHz (about 0.16 nm). For this reason, in the case of using a binary optical intensity modulation signal, it is difficult to set a wavelength interval to 0.16 nm or less.
The multi-valued modulation method is an effective method as the method of making narrow an optical spectral width without reducing a transmission capacity. As described above, according to the multi-valued modulation method, it is possible to realize a large transmission capacity without making narrow the time slot width. For example, when a quaternary code is used, it is possible to realize the same transmission capacity by an optical width spectrum width of half of the case of using a binary code. Thus, the multi-valued modulation method is a method effective for making narrow the wavelength interval in the wavelength multiplexing system.
Moreover, a narrowness of optical spectral width is effective to an influence by wavelength dispersion in an optical fiber transmission. The wavelength dispersion is a difference of propagation time by wavelength, and is a factor of pulse distortion. If the optical spectral width is narrow, a wavelength range included in optical signal becomes narrow; therefore, a pulse distortion becomes small. For this reason, it is expected that the multi-valued modulation method is effective to the influence by chromatic dispersion, which is a problem in the optical fiber transmission.
FIG. 10 is a block diagram showing a configuration 1 of a conventional multi-value modulation apparatus. The multi-valued modulating technology has been disclosed in the document, “Sheldon Waklin and Jan Conradi, “Multi-valued Signaling for Increasing the Reach of 10 Gb/s Light wave Systems”, Journal of Light wave technology, Vol. 17, No. 11, pp. 2235–2248, 1999”. In FIG. 10, the multi-valued modulation signal generating unit disclosed in the above document has been rewritten.
As shown in FIG. 10, the multi-value modulation apparatus comprises binary code generating units 60 and 61 which generates binary code signals having the same amplitude, an attenuator 62 which attenuates an output level of the binary code generating unit 61 to about half, a power synthesizer 63 which power-synthesizes an output of the binary code generating unit 60 and an output of the attenuator 62, and an electric/optical converter (E/O) 64 which converts an output of the power synthesizer 63 into an optical signal.
Operation of the conventional multi-value modulation apparatus will be described below with reference to FIG. 11. (a) indicates an output waveform of the binary code generating unit 60, and (b) indicates an output waveform of the attenuator 62. These signals are added together by the power synthesizer 63; as a result, a quaternary code signal as shown by (c) in FIG. 11 is obtained. The quaternary electric signal is converted into an optical signal having the same waveform by the electric/optical converter 64.
However, according to the configuration shown in FIG. 10, it is difficult to obtain the power synthesizer 63, which adds an electric signal without distortion, and further, it is difficult to obtain the electric/optical converter 64, which converts an obtained quaternary code electric signal into an optical signal without distortion. Many electric/optical converter practically used have a non-linear response characteristic. Therefore, a distortion is generated in conversion. For this reason, a problem arises such that it is difficult to obtain a multi-valued modulation optical signal having a small waveform distortion.
In order to actually obtain a multi-valued modulation optical signal having almost no distortion, for example, as shown in FIG. 12, there has been proposed a method, which does not add the electric signal, but adding an optical signal. FIG. 12 is a block diagram showing a configuration example 2 of the conventional multi-value modulation apparatus. FIG. 12 shows the configuration disclosed in Japanese Patent Application Laid-Open No. 63-5633 (optical multi-valued communication system).
As shown in FIG. 12, the multi-value modulation apparatus comprises optical modulation signal generating units 70 and 71 which output binary intensity modulation signals having different output amplitudes, respectively, and an optical composer 72 which composes binary intensity modulation optical signals output from the optical modulation signal generating units 70 and 71.
Operation of the conventional multi-value modulation apparatus will be described below with reference to FIG. 11. For example, the optical modulation signal generating units 70 outputs a modulation optical signal having a wave form as shown by (a) in FIG. 11, and the optical modulation signal generating units 71 outputs a modulation optical signal having a waveform as shown by (b) in FIG. 11. In this case, it is possible to obtain a quaternary multi-valued modulation optical signal as shown by (c) in FIG. 11 from the optical composer 72. According to this method, the addition of signal is performed by the optical composer. Therefore, a signal distortion is hard to be generated, and an ideal signal waveform is obtained.
However, according to the configuration shown in FIG. 12, there is a limit such that the optical modulation signal generating units 70 and 71 must output different waveform, or a polarized wave must be made orthogonal, in order to prevent a beat noise from generating by the composer of optical signal. In this case, if a signal has different wavelength, it is hard to be applied to a wavelength multiplexing system. Further, a problem arises such that an orthogonality of polarized wave is not always secured in a long distance transmission.
Moreover, in the case of adding two binary code optical signals, the amplitude of one optical signal must be controlled so as to be half of the other optical signal. However, in the above Publication, there is no disclosure relative to the control method. Further, the phase of two binary code optical signals must be controlled so that a timing when amplitude changes is coincident with each other in the case of the addition. However, in the above Publication, there is no disclosure relative to control unit, likewise.